Why water freezes faster after heating

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THIS ARTICLE APPEARS IN NEW SCIENTIST MAGAZINE ISSUE: 3 JUNE 2006

AUTHOR: MARCUS CHOWN

A common chemical process may explain a bizarre property of water that has been a mystery since the time of Aristotle ĘC how hot water can freeze more quickly than cold.

This strange and counterintuitive effect was first observed by the ancient Greek philosopher and was made famous in recent times by a Tanzanian school student called Erasto Mpemba. He noticed that the sugared milk he was using to make ice cream froze more quickly if it started out hot. But what is behind the so-called "Mpemba effect"?

Jonathan Katz of Washington University in St Louis, it's all to do with solutes. "You have to ask yourself: what does heating do to water that makes it easier to freeze?" he says. "The answer is that it precipitates out solutes."

The solutes Katz has in mind are calcium and magnesium bicarbonate, which make most drinking water "hard". When the water is heated, these precipitate out to form the solid scale that "furs" up the inside of a kettle.

Water that has never been heated still contains these solutes. As it freezes, ice crystals form, and the concentration of solutes in the remaining water becomes ever higher up to 50 times as high as normal. This lowers the freezing point of the water, just like salt sprinkled on a road in winter. "The water therefore has
to cool further before it freezes," says Katz.

There is a second, related effect that hampers the freezing of water that has never been heated. The lowering of the freezing point reduces the temperature difference between the liquid and its freezing surroundings. "Since
the rate at which heat is lost from the water depends on this temperature difference, water that has not been heated has greater difficulty losing heat," Katz says.

Katz claims that the two effects combined can perfectly explain why water that has been heated freezes more quickly than water that hasn't. And he makes a prediction that experiments should be able to verify: that the Mpemba effect should be more marked the "harder" the water. "This may explain why not everyone sees it," he says. "Some people are using soft water." "Katz's analysis of the Mpemba effect is deeper and more rigorous than anything else on the subject," says Richard Muller of the University of California at Berkeley. "He has come up with a simple yet I believe correct way to look at a complex phenomenon."

Katz, who worked out the details of the Mpemba effect while adjudicating a student exam, is waiting for someone to do the experiment to test his theory. "It's not difficult but it's not trivial either," he says. "I think it would take a couple of months to do it right."

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